Known cigarette packing machines normally comprise a blank feed device of the type described in U.S. Pat. No. 5,029,834, wherein a substantially vertical feed channel houses a stack of blanks and terminates with a stop surface defined by a frame having a withdrawal opening closed partly by fixed teeth to retain the blanks inside the channel. A suction pickup device engages the withdrawal opening cyclically to withdraw and feed the bottom blank in the stack to a follow-up operating unit on the packing machine.
In blank feed devices of the above type, the blanks are fed along the feed channel by force of gravity, so that the force exerted by the blanks on the stop surface frame depends on the weight of the stack (i.e. on the number of blanks in the stack) and on the friction forces between the blanks and the walls of the feed channel.
In actual use, the number of blanks in the stack inside the feed channel varies continually, on account of the blanks being withdrawn substantially continuously from the feed channel and being loaded into the feed channel in stacks. Moreover, the friction forces between the blanks and the walls of the feed channel also vary continually on account of inevitable minor differences (within given tolerances) in the size of the blanks.
Consequently, the force exerted by the blanks on the stop surface frame also varies continually, as opposed, preferably, to remaining substantially constant at a given value not so high as to prevent the suction pickup device from withdrawing the blanks smoothly, yet not so low as to prevent the blanks from being packed and aligned properly inside the feed channel.
DE-A1-3723589 discloses a device for feeding paper foils, the device comprising a pickup station having a stop roller; a conveyor for feeding a succession or stack of paper foils to the pickup station and along a given horizontal path; pickup means located at the pickup station to withdraw an end paper foil contacting the stop roller; a force sensor connected to the stop roller to determine a value of a thrust exerted by the paper foils on the stop roller; and control means connected to the force sensor and to the conveyor to maintain the thrust constant by regulating a traveling speed of the conveyor. The pickup means comprise a pair of cooperating rollers, which are arranged below the stop roller and engage the end paper foil for feeding same to further processing means.
The known device described above, the stop roller is necessarily arranged contacting a central portion of the surface of the end paper foil to be withdrawn. The stop roller being arranged centrally of the end paper foil implies withdrawing the end paper foil by sliding same along the adjacent paper foil and in contact with the stop roller which, especially when using paper foils with shining surfaces such as cigarette packet blanks, may damage the foils.